Rotary actuators are, of course, well-known devices having utility in diverse applications. A particularly common rotary actuator, or power hinge as it is sometimes called, is comprised of an input including a sun gear which mates with a plurality of planet gears disposed for rotation and circulation thereabout intermediate fixed ring gears. In turn, an output gear is driven which, depending upon the gear ratio, usually moves but a fraction of an arcuate distance in response to a full rotation of the input. Consequently, a rotary input to the sun gear of the output gear and any arcuate displacement of the output gear and any associated linkage member.
Turning to the intended field of utility for the instant invention, applicable requirements are very stringent in respect of those devices associated with a control surface of an aircraft. Unless a control surface is dynamically mass balanced, it must be held rigidly or damped at all times. But, mass balance imposes a severe weight penalty and it is therefore advantageous to strive for a rigidly maintained link with a control surface in order to realize a corresponding weight reduction. When pursuing that objective, multiple redundant devices must be utilized in order to guard against the loss of a rigid link with the control surface. Indeed, a failsafe characteristic as respects but a single device can be of paramount importance due to the obvious adverse consquences of an absence of at least one rigid link to the aircraft control surface under failure conditions.
With the foregoing understanding, it will now be appreciated that conventional rotary actuators may not meet the stringent requirements demanded in this environment. For example, in the event a pinion is stripped or a separator fails, resulting in collapse, the single load path through the device may lack the ability to provide the required stiff link. Back-driving through the actuator could occur under those circumstances; or similar loss of integrity may result in a failure to maintain the mandatory rigid link. Simply including a plurality of conventional actuators is not a practical or workable approach to meeting the multiple redundancy requirements. If one actuator or the input shaft between actuators fails, the linkage from the actuator output arm to the control surface may tend to operate through an overcenter position. Consequently, significant forces can be developed between the operating and the failed actuator and become self-destructive. Yet, to date, the art has not responded to these problems and provided a simple but efficient rotary actuator having failsafe features, whereby the same may find enhanced utility in association with an aircraft control surface. Thus, the need exists for such a device.